This invention relates to fluid-gauging probe assemblies and to tanks including such assemblies.
The invention is more particularly concerned with probes of the capacitive kind for measuring the height of fuel in the fuel tank of a vehicle.
Conventional capacitive probes comprise concentric metal tubes, forming opposite plates of a capacitor, which are mounted rigidly within the tank and which are open to allow fuel to enter between the tubes to a height indicative of the fuel level in the tank. Change in fuel level causes a change in the capacitance of the probe which is used to provide an indication of fuel quantity. Probes of this kind are commonly used in aircraft of the fixed-wing type and in other aircraft, such as, helicopters.
One problem with such probes is that, because of the extremely high axial strength of a metal tube, the probe may pierce the wall of the fuel tank when the aircraft or other vehicle is damages in a crash. Many helicopters now use flexible fuel tanks of a rubber material which is inherently less dangerous in a crash since it can conform to accommodate deformation in the vehicle structure without damage to itself. Such tanks, however, are more likely to be pierced by the fuel probe, leading to leakage of fuel and a consequent fire hazard.
As a result of this, some manufacturers now require the probe to be designed so that it can break or deform to accommodate for change in shape of the fuel tank. One proposal is to weaken the probe at some point along its length by cutting out parts of the wall of the tube. Whilst this is effective in weakening the tube in flexure, that is when it is subjected to a lateral force, it still leaves the tube relatively strong as regards axial compression. There is also the problem that, when the probe breaks, sharp edges are produced at the break which can increase the risk of puncturing of the tank wall.
In an alternative arrangement, the probe is of a telescopic configuration comprising several tubes which can be slid one within the other on compression. This also has several disadvantages in that, because the telescoping tubes overlap one another at their joint, the capacitance of the probe in that region is affected and calibration rendered more difficult. The probe is also relatively strong in flexure and there is the problem that corrosion or fouling of the probe might prevent free sliding of the tubes within one another on axial compression. A telescoping arrangement is also more expensive since it is not generally possible to obtain tubes from stock which are a sliding fit within one another; instead the tubes must be fabricated especially.